This project focused on: (1)\differentiation and origin of natural killer (NK) cells; (2)\mechanism of killing mediated by NK cells; and (3)\regulatory pathways in natural killing. We have shown that precursors of large granular lymphocytes (LGL) exist in peripheral blood and they can be induced to differentiate to LGL by contact with cultured lymphoma blasts and subsequent incubation on monolayers of macrophages. A new monoclonal antibody, NK-9, was developed. It recognizes both mature LGL as well as precursors of LGL both in peripheral blood and bone marrow. The NK-9-positive precursor pool also contains precursors of cytotoxic T cells and activated killer cells (NK-like cells), suggesting a common origin of NK cells and T cells. In regard to the mechanism of killing, we showed that F-actin and vinculin in LGL polarize towards the contact area between NK-cells and target cells, whereas other cytoskeletal proteins remain unpolarized. Detergent treatment of conjugates with labelled LGL revealed a 95 kilodalton protein that partially inhibits the conjugate formation and thus is associated with the target recognition structures of NK cells. In terms of the regulatory pathways in natural killing, we studied various patient groups to identify altered expressions of NK activity. We also analyzed inhibitory substances recovered from tumor patient ascites fluids and supernatants of cultured tumor cells. (SR)